During starting processes of vehicle drive-trains constructed with conventional automatic variable-speed transmissions, when a driver wishes to start off and actuates the gas pedal there occur only slight delays which the driver can perceive only with difficulty or even not at all. With automatic variable-speed transmissions, already during the creation of the force linkage by way of a hydrodynamic torque converter, torque is provided in the area of a drive output of a vehicle drive-train. As a rule this is enabled by increasing the transmission capacity of a clutch or the like. By increasing the transmission capacity of the clutch, the rotational speed of the previously freely rotating turbine of the hydrodynamic torque converter is reduced toward zero whereby in the area of the torque converter a corresponding speed ratio is produced. The torque taken up by a pump impeller wheel of the hydrodynamic torque converter and supplied by a drive mechanism functionally connected thereto is then delivered toward the drive output by the turbine of the torque converter as a function of the torque conversion taking place in the area of the torque converter.
The amount of torque delivered by the turbine cannot exceed the torque produced by a starting element, which depends on a gear ratio between the turbine and the starting element. A vehicle constructed with such a vehicle drive-train is accelerated in accordance with the gear ratio engaged in the area of the automatic variable-speed transmission and the torque that results from the gear ratio in the area of the drive output by virtue of the interplay between the drive machine, the hydrodynamic torque converter and the starting element.
In contrast, in the case of continuously-variable power-branched transmission systems, the force flow between a drive mechanism and a drive output of a vehicle drive-train is produced in the area of one or more shifting elements and then a transmission ratio of such a transmission system is adjusted from infinite to long.
However, this procedure impairs the spontaneity of a vehicle drive-train to an undesired extent, since operating times between the moment when a driver generates a command for a starting process and the moment when the vehicle actually starts off in the desired driving direction can be quite long, and constitute time delays that are clearly perceptible by the driver.
Particularly in the case of working machines, time-delayed responses to a driver's commands are not acceptable. This applies especially to wheel loaders and forestry machines. The use of continuously-variable power-branched transmissions in vehicle drive-trains of such working machines leads to time delays, above all during starting processes.